Demands have been increasingly made to improve fuel efficiency of automobiles, from the viewpoint of energy saving. For energy saving, automobile body weights should be reduced, and materials for use in door impact beams and other reinforcing members of automobiles should have higher tensile strengths. The materials should also excel in shock absorptivity so that the doors less protrude into cabin upon impact to thereby protect occupants. In addition, they should be able to absorb impact energy satisfactorily at low temperatures, because such automobiles may be used and undergo impact accidents in cold regions.
Steel pipes composed of steels with a martensite microstructure have been widely used as such high-strength electric resistance welded steel pipes. United States Patent Application Publication No. 20050034795 discloses a technique relating to a steel pipe composed of a high-strength steel and having a reduced yield ratio, and a method of manufacturing the steel pipe. According to this technique, the yield strength of the steel pipe is determined under such a stress as to yield a 0.1% permanent strain. The document mentions that the properties of the steel pipe can be accurately determined according to this method, because an absorbable energy upon impact is underestimated when the yield strength is determined under such a stress as to yield a 0.2% permanent strain as specified in Japanese Industrial Standards (JIS). The steel pipe according to this technique, however, has a low yield strength, thereby undergoes local buckling at low loads upon bending, fails to have a high resistance to buckling loading (crushing strength), and less absorbs energy upon bending. As a result, it is difficult to absorb much energy upon impact.
Japanese Unexamined Patent Application Publication (JP-A) No. 2001-164338 discloses a technique for yielding a high-strength steel pipe by subjecting a steel pipe having an adjusted composition to induction hardening. The resulting steel pipe, however, has not been subjected to a treatment after quench hardening, has a low yield ratio of about 75% as determined under a 0.2%-proof stress, and fails to attain a high resistance to buckling loading (to have a high crushing strength).
JP-A No. H04-180537 discloses a technique of providing a steel pipe having a tensile strength TS of 100 kgf/mm2 or more and a yield ratio of 80% or more. In this technique, a steel of an austenite structure is quenched to give a martensite microstructure, and this steel is tempered at high temperatures of 200° C. to 450° C. The resulting steel pipe, however, has a low strength of about 120 kgf/mm2, although it has a high yield strength.
JP-A No. H04-276018 discloses a technique, in which a steel pipe having an adjusted composition is quenched, overaged at a temperature of 150° C. to 450° C. to give ferrite and martensite microstructures, processed to 1% to 30%, and bake-hardened. The resulting steel pipe, however, has a volume fraction of martensite of about 70% and a low strength in terms of tensile strength of 170 kgf/mm2.